Hello, my name is Dan Harmon, and I am the product marketing engineer for Texas Instruments current sense amplifiers. In session 2 of this series, I will address design considerations that must be taken into account when implementing your current sensing with a current shunt monitor, also called a current shunt amplifier or current sense amplifier.
First, a quick recap of what makes a current sense amplifier special. First is the unique floating input stage, which allows for common mode voltages in excess of the supply rail. And second is the integrated precision gain networks that maximize accuracy.
On the right is a list of questions that you should review before starting your design. The following slides will look at each topic.
The concept of low-side or high-side was introduced in session one. A low-side implementation has the shunt resistor placed between the load and system ground. This is the most common method to monitor currents since the common mode voltage is essentially 0 volts. However, this also disturbs the ground seen by the load, as well as preventing the detection of load shorts to ground.
A high-side implementation has the shunt resistor placed between the supply voltage and the load, resulting in the common mode voltage being essentially equal to the bus voltage. In many applications this can be well above the supply rail of the amplifier. High-side sensing allows for the system to not have any ground disturbance and to identify ground shorts.
Let's revisit the common mode voltage range. If you are measuring on the high-side, the common mode voltage is typically the bus voltage monitored. You will need to choose a current shunt amplifier that includes this rail, as well as any voltage margin required by the application within its common mode voltage range. However, for low-side measurements, the common mode voltage is essentially 0 volts, so you need to choose a device that includes zero in its common mode range.
I'm showing here two data sheet extracts. The top device can only be used in high-side implementations with the 2.7 volt minimum common mode limitations shown, while the bottom device can be used for either high or low side applications, as the common mode range extends below ground. While the current range itself does not derive any specific amplifier specifications, when you look at the range of current, the signal conditioning that follows, and the R shunt value, you can derive two key specifications, the required gain of the stage as well as the minimum differential voltage requirement of the stage.
Now we will take a look at the concept of current flow directionality. In many applications the current may flow in both directions. For example, is a battery charging or discharging. In order to determine which direction the current is flowing, the current sense amplifier must have an additional input pin that can be used to divide the output range into a system sinking current or system sourcing current as shown in the diagram.
How the current information is being used is the next key decision that will help drive device selection. In other words, what is the next link in the signal conditioning chain? One item to determine is output type, analog current, analog voltage, digital, or alert.
If the value of the current is used purely as feedback or control of another analog device, then sticking with analog output makes the most sense. If the information is going to be processed by a microcontroller, then using a digital output device is the best choice. This will also allow you to potentially save additional signal conditioning cost. If only comparison to an overcurrent threshold is required, then a simple alert output may be all that is needed.
As discussed previously, the required gain of the system will be determined by the combination of how it is being used, as well as the current range multiplied by the shunt value. If the analog output is going to be used as feedback or going to an analog to digital converter, there is a range that must be matched to maximize performance. This will set the gain of the amplifier stage.
There are many application requirements that will help determine the best current shunt monitor used for any given system implementation. This session has introduced five of these topics. While it may be the most important question, the required accuracy and maximizing the accuracy were not addressed in this session, but is the focus of the balance of the series.
For more information on TI's current sense amplifiers, please watch the remainder of the Current Sense video series, as well as go to www.ti.com/currentsense. Thank you for your time today. 大家好，我叫 Dan Harmon， 是德州仪器 (TI) 的 电流感应放大器 产品营销工程师。 在本系列的 第 2 节，我将 介绍在通过 电流分流监控器 （也叫电流分流放大器 或电流感应放大器） 实现电流 感应时 需要考虑的 设计注意事项。 首先，我们快速回顾一下 电流感应放大器有哪些 特别之处。 第一是独特的 浮点输入级， 它允许共模 电压超过 电源轨。 第二是集成的 精度增益网络， 能够实现最大限度的精度。 右侧列出了 一些您在 开始设计前 应考虑的问题。 在下面的幻灯片中， 我将逐个介绍各个主题。 第 1 节介绍了 低侧或高侧 的概念。 低侧实现 在负载和 系统接地之间 放置分流电阻。 这是监控电流的 最常用方法， 因为共模电压 实质上是 0 伏。 但是，这也会影响 负载所见的接地， 并阻止对负载 短路接地 的检测。 高侧实现 在电源电压 和负载之间放置 分流电阻， 使共模 电压 实质上等于 总线电压。 在许多应用中， 这可能会显著 高于放大器 的电源轨。 高侧感应 可以使系统 不受任何 接地干扰 并能识别接地短路。 我们再来回顾一下共模 电压的范围。 如果您在测量 高侧， 共模电压 通常是 所监控的总线电压。 您将需要选择一个 包含该电源轨 的电流分流放大器， 以及应用所需的 在其共模电压范围内 的任何电压裕量。 但是，对于 低侧测量， 共模电压 实质上是 0 伏， 因此您需要选择 一个共模电压范围 包含 0 伏的器件。 我这里展示的是从数据表中 抽取的两个器件。 顶部器件只能用于 高侧实现， 并具有所示的 2.7 伏 最小共模电压限制， 而底部器件 既可用于 高侧应用也可 用于低侧应用， 因为共模范围 扩展至低于接地值。 电流范围 本身不会 得出任何特定的 放大器规格； 当您查看电流范围、 随后的电流 调节以及 R 分流值时， 您可以得出两个关键 规格，即所需的 级增益以及 级的最小差分 电压要求。 现在，我们来看看 电流方向 的概念。 许多应用都可以 实现双向电流。 例如，蓄电池是在 充电还是放电？ 为了确定 电流流向， 电流感应 放大器 有一个额外 的输入引脚， 可以用于将 输出范围分成系统 灌电流或 系统拉电流， 如图所示。 如何使用 电流信息 是下一个将 有助于推动器件选择的关键 决定。 换言之，在信号 调节链中，下一环 是什么？ 要确定的一项是 输出类型、 模拟电流、模拟 电压、数字或警报。 如果电流 的值完全用于 反馈或对另一个 模拟器件的控制， 则坚持使用模拟输出 是最合理的选择。 如果将由 微控制器 处理信息， 则使用数字输出器件 是最佳选择。 这也使您 有可能节省 额外的信号 调节费用。 如果只需与 过流阈值进行比较， 则需要的 可能只是简单 的警报输出。 前面讨论过， 系统所需的增益 是由其使用方式 以及电流范围 乘以分流值 所得的结果共同 决定的。 如果模拟输出 将用作反馈 或进入模数 转换器， 则必须与某一 范围相匹配才能 最大程度地提高性能。 这将设置放大器 模块的增益。 有许多应用 要求将会 帮助确定用于 任何特定系统实现 的最佳电流 分流监控器。 本节介绍了这些主题 中的五个主题。 虽然所需的 精度和如何 最大程度地提高精度 是最重要的问题， 但本节 并未涉及， 不过，这是系列视频剩余 部分的重点内容。 有关 TI 的电流感应 放大器的更多信息， 请观看电流感应 视频系列的其余部分， 并访问 www.ti.com/currentsense。

Details

Date:
May 12, 2015

The Getting Started with Current Sense Amplifiers series helps engineers learn how to to maximize the performance achieved when measuring current with a current sense amplifier (also called a current shunt monitor). Session two reviews important considerations when designing with a current shunt monitor.